So far, the preparation of thermo-crosslinkable and/or thermoplastic elastomer blends, in particular of vulcanizable rubber blends, was generally effected, either:
or preferably
In variant (a), the calender comprised two parallel, horizontally mounted heatable and coolable hollow rolls. For carrying out mixing, the crude rubber was put onto the rolls in the form of coarse pieces, and was worked until a smooth sheet was obtained. Thereafter, the processing and vulcanization adjuvants were batchwisely added, and the rolled sheet were repeatedly homogenized by cutting and finally cooled with water in order to prevent them from prematurely vulcanizing.
The internal mixer according to variant (b) comprised a closed mixing chamber and two heatable and coolable mixing paddles arranged in said mixing chamber. This feature did provide, it is true, a higher mixing speed and a more intensive mixing effect than variant (a). However, due to the applied speed of rotation, the crude rubber blend in the internal mixer was so strongly heated that no crosslinking reagents could be added. Therefore, it was often necessary either
or alternatively
Besides the effectiveness of the mixing aggregate, the order and the moment of addition of the adjuvants have a decisive influence on the elastomer quality. Thus, it was necessary to first add difficultly mixable plasticizers which do not initiate crosslinking, and to add vulcanizing agents at the end only.
So far, the attempts for achieving a continuous mixing in mixing extruders failed for the abovementioned compulsion of observing this order. Calculations show that a spindle length of the order of 40.degree. D would be necessary (D being the spindle diameter). This is technically difficult to realize and economically unattractive.
Furthermore, the use of plasticized elastomers or the addition of large quantities of plasticizers was critical since the shearing forces produceable in the mixture were no longer sufficient for a regular mixing.